Which of the following will have least hindered rotation about carbon -carbon bond?

Options

A. Ethane
B. Ethylene
C. Acetylene
D. Hexachloroethane

Explanation

To determine which compound has the least hindered rotation about the carbon-carbon bond, we need to analyze the structures and types of bonds present in the given compounds.

### Step-by-Step Solution:

1. **Identify the Compounds**: The question provides four compounds: ethane, ethylene, acetylene, and hexachloroethane.

2. **Analyze the Type of Bonds**:
- **Ethane (C2H6)**: Contains a single bond (C-C).
- **Ethylene (C2H4)**: Contains a double bond (C=C).
- **Acetylene (C2H2)**: Contains a triple bond (C≡C).
- **Hexachloroethane (C2Cl6)**: Contains a single bond (C-C) with bulky chlorine atoms.

3. **Understand Bond Rotation**:
- **Single Bonds**: Rotation around single bonds is generally free unless hindered by steric effects (size of substituents).
- **Double and Triple Bonds**: Rotation around double and triple bonds is restricted due to the nature of these bonds.

4. **Evaluate Steric Hindrance**:
- **Ethane**: Has hydrogen atoms (small groups) attached to the carbon atoms, allowing for easy rotation.
- **Ethylene**: Restricted rotation due to the double bond.
- **Acetylene**: Restricted rotation due to the triple bond.
- **Hexachloroethane**: Although it has a single bond, the presence of six chlorine atoms (bulky groups) creates significant steric hindrance, making rotation more difficult.

5. **Conclusion**: Ethane, with its small hydrogen substituents, will have the least hindered rotation about the carbon-carbon bond compared to the other compounds.

### Final Answer:
**Ethane (C2H6)** has the least hindered rotation about the carbon-carbon bond.

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